This invention pertains to apparatus for cutting sheet metal or like work by means of a jet of highly pressurized water containing particles of an abrasive material. Such a jet of water containing abrasive particles is herein referred to as an abrasive water jet.
The cutting of metal or like materials by an abrasive water jet per se is not new and has been extensively practiced for some time now. As the abrasive particles there have been employed materials such as silica sand, cast iron grit, or powdered garnet or alumina. The average size of such abrasive particles has heretofore ranged from 0.2 to 0.8 millimeters.
The conventional abrasive water jet cutting practice has been such that highly pressurized water and "dry" abrasive particles are separately delivered to an abrasive water jet nozzle assembly. The abrasive particles merge into the stream of pressurized water within the nozzle assembly. The pressurized water containing the abrasive particles emerges from the nozzle assembly in the form of an abrasive water jet directed against the work. The abrasive water jet cuts the work as the nozzle assembly and work are moved relative to each other.
An objection to this conventional practice is that the abrasive particles are fed in the "dry" state to the nozzle assembly and, in consequence, cannot possibly be reduced in size to such an extent as can be desired. The use of such coarse abrasive particles (from 0.2 to 0.8 millimeters in average size) is undesirable for more reasons than one. First, they give rise to the rapid wear of the nozzle tip and other parts of the nozzle assembly that are exposed to the abrasive particles. Second, the width of cut (kerf) created in the work is very large, usually larger than the orifice diameter of the nozzle tip. An additional problem is the production of burrs on the back of the work. Consequently, the abrasive water jet cutting as heretofore practiced has required subsequent machining operations in applications where close tolerances and fine finishes are important.
U.S. Pat. No. 3,972,150 to Hart teaches the use of a slurry made of a mixture of abrasive material and water for cutting a work. The slurry is induced by and merges into high pressure water jets and is directed against the work to cut the same. Again, the abrasive material used is made of coarse abrasive particles, so that fine cutting with a small width of cut and without the production of burrs cannot be realized.